Studies showed that the aberrant of Period2 (PER2) gene, a core component of circadian oscillator, is responsible for rhythm alteration of physiological activities including growth, metabolic and endocrine regulation. This study employed bovine mammary epithelial cells (BMEC) as a model to investigate the effect of the suppression of PER2 on cell proliferation, apoptosis and cell cycle progression. BMEC cells were established by enzymatic digestion of mammary tissue from mid-lactation cows and cultured in growth medium (DFEM/F12 as basis). The fourth passage BMEC were transiently-transfected with siRNA specific for PER2 to downregulate transcription. After incubating for 24 h and 30 h, the alterations in proliferative activity of BMEC by cell counting Kit-8, apoptosis and cell cycle distribution by flow cytometry, DNA fragmentation by agarose gel electrophoresis and mRNA expression alterations in the important genes related to cell cycle network and circadian rhythm by qPCR were all detected. Statistical analysis of data were performed using an independent sample t-test module (SPSS16.0). The results showed that cell proliferation was increased (P < 0.05) while cell apoptosis rate declined (P > 0.05), the DNA fragmentation did not significantly respond to PER2 silence and the number of cells in the G1/G0 phase was decreased both for 24h and 30h transfection (P < 0.01). The gene expression levels of cyclin D1, CDK2, p21, PER1 and BMAL1 were dramatically decreased (P < 0.05), whereas the CDK4 mRNA level was significantly increased (P < 0.05) compared with control cells after 24h incubation. In conclusion, the present study supports that PER2 regulates proliferation, apoptosis and cell cycle progress of BMEC and effects expression of core circadian clock gene BMAL1 at the transcriptional level. The multiple implications of our findings for further studies lies in understanding how PER2 regulates bovine mammary gland development and circadian mammary organic functions.